1. Field of the Invention
The present invention relates to a heat-sensitive recording medium.
2. Description of the Related Art
Conventional heat-sensitive recording mediums are constituted of a heat-sensitive color-developing layer provided on a support such as a paper sheet, a film, etc., in which the heat-sensitive color developing layer is composed of a two-component color-developing system consisting of a colorless or light-colored electron-donating dye-precursor (hereinafter referred to as a "leuco dye"), and electron-acceptor (hereinafter referred to as a "developer"), and additionally a binder, a sensitivity-improving agent, a lubricating agent, a filler, and other auxiliary agents dispersed in the system. Such a material is disclosed, for example, by Japanese Patent Publications No. Sho 43-4160 (1968), and No. Sho 45-14039 (1970).
Such two-component color-developing type of heat-sensitive recording mediums, which are capable of easily developing a color image by heating with a heating element such as a thermal head, a heating pen, etc., have the features below.
(1) The primary color-development eliminates the steps of development and fixing of images, PA1 (2) The supply of toner or the like is not necessary, and the maintenance of the recording machine is easy, PA1 (3) The quality of the medium resembles that of general purpose paper, and does not cause feeling of unnaturalness, when paper is used as the support, PA1 (4) The density of the developed color is high, so that sharp image is obtained, and PA1 (5) Various color tones can easily be developed by changing the kind of the leuco dye.
Because of these advantages, the two-component color-developing type of heat-sensitive recording mediums are most commonly employed among heat-sensitive recording mediums. Recently, in particular, the use thereof is remarkably increasing in application fields of facsimiles, printers, instrumentation recorders, labels, and others.
In the field of recent facsimile communication, for example, increase of recording speed is intended with the speed-up of the communication. Accordingly, heat-sensitive recording paper is strongly demanded to be improved in heat responsiveness such that sufficient color density be achieved with less thermal energy.
For the improvement of the heat responsiveness, studies have been made on various development-sensitivity improving agents. However, accompanied with the improvement of the heat responsiveness, undesirable tendency of texture fogging increases. At the moment, no satisfactory heat-sensitive recording medium has not been obtained which satisfies the needs both for high color-developing sensitivity and less texture fogging.
The components mentioned below are contained in the heat-sensitive color-developing layer of the heat sensitive recording medium generally proposed or widely used.
The leuco dyes are exemplified by the following compounds:
Triarylmethane compounds, including
3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide, (i.e., Crystal Violet lactone), PA0 3,3-bis(p-dimethylaminophenyl)phthalide, PA0 3-(p-dimethylaminophenyl)-3-(1,3-dimethylindol-3-yl)phthalide, PA0 3-(p-dimethylaminophenyl)-3-(2-methylindol-3-yl)phthalide, etc. PA0 4,4'-bis-dimethylaminobenzhydrinbenzylether, PA0 N-halophenyl-leucoauramine, PA0 N-2,4,5-trichlorophenyl-leucoauramine, etc. PA0 rhodamine-B-anilinolactam, rhodamine (p-nitrino)lactam, PA0 2-(dibenzylamino)fluoran, PA0 2-phenylamino-6-diethylamino-fluoran, PA0 2-(0-chloroanilino)-6-diethylaminofluoran, PA0 2-(3,4-dichloroanilino)-6-diethylaminofluoran, PA0 2-anilino-3-methyl-6-piperidinofluoran, PA0 2-phenyl-6-diethylaminofluoran, PA0 2-anilino-3-methyl-6-dimethylaminofluoran, PA0 2-anilino-3-methyl-6-N-methyl-N-ethylaminofluran, PA0 2-anilino-3-methyl-6-N-methyl-N-(isopropyl)aminofluoran, PA0 2-anilino-3-methyl-6-N-methyl-N-pentylaminofluoran, PA0 2-anilino-3-methyl-6-N-methyl-N-cyclohexylaminofluoran, PA0 2-anilino-3-methyl-6-diethylaminofluoran, PA0 2-anilino-3-chloro-6-dimethylaminofluoran, PA0 2-anilino-3-methyl-6-N-ethyl-N-isoamylaminofluoran, PA0 2-anilino-3-methyl-6-N-methyl-N-isoamylaminofluoran, PA0 2-anilino-3-chloro-6-diethylaminofluoran, PA0 2-anilino-3-chloro-6-N-methyl-N-ethylaminofluoran, PA0 2-anilino-3-chloro-6-N-methyl-N-(isopropyl)aminofluoran, PA0 2-anilino-3-chloro-6-N-methyl-N-pentylaminofluoran, PA0 2-anilino-3-chloro-6-N-methyl-N-cyclohexylaminofluoran, PA0 2-anilino-3-methyl-6-N-ethyl-N-pentylaminofluoran, PA0 2-anilino-3-chloro-6-N-ethyl-N-pentylaminofluoran, PA0 2-(p-methylanilino)-3-methyl-6-dimethylaminofluoran, PA0 2-(p-methylanilino)-3-methyl-6-diethyaminofluoran, PA0 2-(p-methylanilino)-3-methyl-6-N-methyl-N-ethylaminofluoran, PA0 2-(p-methylanilino)-3-methyl-6-N-methyl-N-(isopropyl)aminofluoran, PA0 2-(p-methylanilino)-3-methyl-6-N-methyl-N-pentylaminofluoran, PA0 2-(p-methylanilino)-3-methyl-6-N-methyl-N-cyclohexylaminofluoran, PA0 2-(p-methylanilino)-3-methyl-6-N-ethyl-N-pentylaminofluoran, PA0 2-(p-methylanilino)-3-chloro-6-dimethylaminofluoran, PA0 2-(p-methylanilino)-3-chloro-6-diethylaminofluoran, PA0 2-(p-methylanilino)-3-chloro-6-N-methyl-N-ethylaminofluoran, PA0 2-(p-methylanilino)-3-chloro-6-N-methyl-N-(isopropyl)aminofluoran, PA0 2-(p-methylanilino)-3-chloro-6-N-methyl-N-cyclohexylaminofluoran, PA0 2-(p-methylanilino)-3-chloro-6-N-methyl-N-pentylaminofluoran, PA0 2-(p-methylanilino)-3-chloro-6-N-ethyl-N-pentylaminofluoran, PA0 2-anilino-3-methyl-6-N-methyl-N-furylmethylaminofluoran, PA0 2-anilino-3-ethyl-6-N-methyl-N-furylmethylaminofluoran, etc. PA0 benzoleucomethylene blue, PA0 p-nitrobenzyl leucomethylene blue, etc. PA0 3-methyl-spiro-dinaphthopyrane, PA0 3-ethyl-spiro-dinaphthopyrane, PA0 3,3'-dichloro-spiro-dinaphthopyrane, PA0 3-benzyl-spiro-dinaphthopyrane, PA0 3-methyl-naphtho-(3-methoxybenzo)-spiropyrane, PA0 3-propyl-spiro-dibenzopyrane, etc. PA0 2,2-bis(4'-hydroxyphenyl)propane, PA0 2,2-bis(4'-hydroxyphenyl)pentane, PA0 2,2-bis(4'-hydroxy-3',5'-dichlorophenyl)propane, PA0 1,1-bis(4'-hdyroxyphenyl)cyclohexane, PA0 2,2-bis(4'-hydroxyphenyl)hexane PA0 1,1-bis(4'-hydroxyphenyl)propane, PA0 1,1-bis(4'-hydroxyphenyl)butane PA0 1,1-bis(4'-hydroxyphenyl)pentane, PA0 1,1-bis(4'-hydroxyphenyl)hexane, PA0 1,1-bis(4'-hydroxyphenyl)heptane, PA0 1,1-bis(4'-hydroxyphenyl)-2-methyl-pentane, PA0 1,1-bis(4'-hydroxyphenyl)-2-ethyl-hexane, PA0 1,1-bis(4'-hydroxyphenyl)dodecane, PA0 3,3-bis(4'-hydroxyphenyl)pentane, PA0 1,2-bis(4'-hydroxyphenyl)ethane, PA0 1,1-bis(4'-hydroxyphenyl) sulfide, PA0 1,1-bis(3'-allyl-4'-hydroxyphenyl) sulfone, PA0 1,1-bis(4'-hydroxyphenyl) sulfone, PA0 4-hydroxy-4'-isopropoxydiphenyl sulfone, PA0 1,1-bis(4'-hydroxyphenyl) ether, PA0 2,2-bis(4'-hydroxy-3',5'-dichlorophenyl)butane, PA0 phenyl 2,4-dihydroxybenzoate, PA0 phenyl 2,4-dihydroxy-4'-methyl-benzoate, PA0 phenyl 2,4-dihydroxy-4'-chlorobenzoate, PA0 phenyl 2,4-dihydroxy-6-methylbenzoate, PA0 phenyl 2,4,6-trihydroxybenzoate, PA0 phenyl 2,4-dihydroxy-6,4'-dimethylbenzoate, PA0 phenyl 2,4-dihydroxy-6-methyl-4'-chlorobenzoate, PA0 benzyl 2,4-dihydroxybenzoate, PA0 benzyl 2,4-dihydroxy-4'-methylbenzoate, PA0 benzyl 2,4-dihydroxy-4'-chlorobenzoate, PA0 benzyl 2,4-dihydroxy-6-methylbenzoate, PA0 benzyl 2,4,6-trihydroxybenzoate, PA0 benzyl 2,4-dihydroxy-6,4'-dimethylbenzoate, PA0 benzyl 2,4-dihydroxy-6-methyl-4'-chlorobenzoate, PA0 ethyl 4-hydroxybenzoate, PA0 propyl 4-hydroxybenzoate, PA0 isopropyl 4-hydroxybenzoate, PA0 benzyl 4-hydroxybenzoate, PA0 benzyl 4-hydroxy-4'-chlorobenzoate, PA0 benzyl 4-hydroxy-4'-methylbenzoate, PA0 benzyl 4-hydroxy-4'-ethylbenzoate, etc.
Diphenylmethane compounds, including
Xanthene compounds, including
Thiazine compounds, including
Spiropyrane compounds, including
The color developing agent used in combination with the leuco dye to develop the leuco dye in the heat-sensitive color developing layer in the heat-sensitive recording medium is an organic acidic substance which is solid at a normal temperature and is thermally fusible. The examples are phenol compounds, aromatic carboxylic acid compounds, novolak type phenol resins, and the like. Typical examples are bisphenol derivatives, hydroxybenzoic acid esters, salycylic acid esters, etc., and inorganic acids such as activated clay and active silica.
Among these color developers, typically, some examples of phenol compounds and aromatic carboxylic acid compounds are as follows:
A sensitizing agent (or color-development-improving agent) may be added in the heat-sensitive color-developing layer of the heat-sensitive recording medium, in addition to the above-mentioned leuco dye and the color-developing agent. The sensitizing agent melts at a relatively low temperature to cause the color reaction between the leuco dye and the color developing agent, thereby serving to increase color-developing sensitivity. Examples of known sensitizing agents for such purpose are as below: waxes and nitrogen-containing compounds such as stearic amide, stearic methylenebisamide, oleic amide, palmitic amide, coconut oil fatty acid amide, etc., carboxylic esters, naphthol derivatives, naphthoic ester derivatives, benzoic ester derivatives, and the like.
As described above, various substances are known as the components in heat-sensitive color-developing layers of the heat-sensitive recording mediums. Heretofore, comprehensive studies have been made on heat-sensitive recording mediums which meet the technical requirements, for example, as are demanded typically in high-speed facsimile and energy-saving type facsimile, for high color-developing sensitivity with uniformity of printed letters and reproducibility of images, and which have a constitution for less texture fogging with higher color-developing sensitivity. As the results, various component combinations have been proposed and practiced.
However, the mechanism of color development has not elucidated in detail, and further, the kinds of leuco dyes, color-developing agents, and sensitizing agents are diversified, so that it is not easy to select the satisfactory combination.
Under such a situation, the inventors of the present invention noticed the dye precursor as described below which has fluoran skeleton to form a leuco dye which will not cause irregularity of letter print but will exhibit high reproducibility. Further the inventors of the present invention investigated comprehensively the specific constitution of a heat-sensitive color developing layer having component composition highly compatible especially with such dyes, and completed the present invention.